MAXON—STUDIES OF TROPICAL AMERICAN FERNS. 135 
marked species had been strangely confused with a nearly or quite 
sterile, incised, leafy form of true A. trichomanes which occurs in 
Europe and has been found once or twice in the eastern United States. 
Asplenium vespertinum is, however, but one of a group of rather 
closely related species, well represented in the American tropics and 
mostly marked by excellent characters, whose relationship and limi- 
tations are not well understood. In view of this fact it has seemed 
desirable to bring together the following notes, and to publish a key 
by means of which the several species may be identified. To this end 
the writer has examined critically the specimens in most of the larger 
American herbaria and has had also the benefit of specimens and 
data obtained in field work over a large part of the area covered. 
As must often be the case, the number of species to be recognized has 
decreased materially as specimens and data have accumulated. 
Asplenium heterochroum and A. castanewm will serve as excellent 
examples. The former species is known chiefly from Bermuda and 
Florida specimens which latterly have been called A. muticum; and 
only upon the very recent collection of adequate Cuban material 
has it been found that Kunze’s A. heterochroum, described from imper- 
fect Cuban specimens and nearly lost sight of since its publication in 
1834, really represents the same species in slightly different form, the 
name heterochroum therefore applying to the whole. In the case of 
A. castaneum there has been not only a recent redescription under the 
name A. rubinum, but also a very general failure to note the unusu- 
ally wide extremes of leaf form within the species, the difference 
amounting almost to the development of two types of fertile fronds, 
as explained later. 
Considering the proneness of ferns to extreme variation, it must 
be apparent that, in the case of many old as well as new or little 
known species, the advantage to be gained from studying a large 
series of specimens is very great, since only in this way will it be 
possible to determine the extent not only of unusual variation, but 
even of the normal fluctuation which is inherent in nearly every 
species. Besides variations of the latter type, relatively and actually 
very great in ferns, unusual attention must be given to transitional 
states which may commonly be correlated with geographical dis- 
tribution. With large series at hand extreme variants are not so 
likely to be described as new species. For the sake of determining 
distribution and relationship, also, unusually extensive collecting is 
necessary in so variable a group. In no other way will it be possible 
to know tropical ferns so thoroughly as we know those of temperate 
regions, the latter from our constant observation of them in the 
field and herbarium being more or less familiar in all their phases. 
We often forget that tropical ferns offer a similar or even greater 
range of variation, and that the chance collecting of a few specimens 
